Infrared thermopile sensor arrays are known which can be produced in different varieties using silicon micromachining technology. In this case, a thin membrane with thermoelements arranged thereon using thin-film technology is usually situated in the center of each sensor cell. Said membrane is situated above a cavity in the underlying silicon substrate.
The thermoelements have so-called “hot” and “cold” contacts, wherein the “hot” contacts are situated on the central part of the membrane, the absorber region, while the “cold” contacts are positioned on the edge of the silicon substrate (pixel). The central part of the membrane containing the absorber and the edge of the silicon substrate serving as a heat sink are connected to one another via thin webs (beams).
The absorption of infrared radiation for the most part takes place in the central region of the membrane. Said region is (in particular in high density arrays) significantly smaller than the size of the pixel.
This has two major disadvantages, since part of the infrared radiation (IR radiation) to the pixel is not used, as a result of which the achievable resolution is reduced.
Secondly, small hot spots (objects or persons to be detected or to be measured), the image of which is incident through the optical system on the edge region of the pixel outside the central region, do not make a sufficient signal contribution and are “overlooked”.
A description is given below of such a solution for a thermopile array from the prior art with reference to EP 2 348 294 or US 2003/0054179 A1. This involves a thermal IR sensor in a housing with a radiation entrance optical system and a chip with thermoelements on a central thin membrane, which is stretched above a frame-shaped carrying body having a good thermal conductivity. What is disadvantageous here is that the absorbent central region is significantly smaller than the total area of a pixel. A multilayer radiation detector layer, which can be constructed by means of conventional processes of circuit production, is situated on the membrane. One major disadvantage here is that only 70% absorption is possible on account of the multilayer structure of a semiconductor circuit stack.
The carrier substrate is hollowed out below the sensor structure, which is achieved by means of a wet-chemical etching method (surface micromachining), oblique walls being produced.
If the thermal sensor is not operated under high vacuum, then the heat conduction of the residual gas or of the filling gas in the sensor housing reduces the achievable temperature difference between the “hot contacts” on the absorber region and the “cold contacts” on the heat sink (carrier substrate).
If the absorbed IR radiation produces a smaller temperature difference, the achievable sensitivity of the sensor cell also decreases.
Kanno, T. et al. (NEC Corp.): “Uncooled focal plane array having 128×128 thermopile detector elements” in B. Andersen (Ed.), Infrared Technology, Proc. SPIE 2269, Vol. XX, San Diego, July 1994, pages 450-459, specify a monolithic thermopile sensor array in which the sensor elements are produced using a surface micromachining technology with a sacrificial layer.
Once again the central part with the absorber layer is significantly smaller than the size of the pixel. The distance between sensor structure and heat sink is significantly smaller than the substrate thickness itself. The solution allows a relatively good resolution only for the case where the sensor chip is encapsulated in the high-vacuum-tight housing. With cost-effective housing constructions under low residual gas pressure, or with a filling gas, sufficiently high sensitivities cannot be achieved.
DE 693 29 708 T2 or EP 0 599 364 B1 is concerned with a production method for infrared radiation sensors in which the sensitivity is improved by the use of a vacuum housing or a housing filled with a gas having only slight thermal conductivity.
The radiation sensor has wet-etched, oblique etching pit walls. Between baseplate and substrate there is a ventilation gap that preferably serves for pressure equalization between the region above and below the membrane. The absorber region here is likewise significantly smaller than the dimensioning of a pixel.
HORIBA product information: “8×8 element thermopile Imager”; in Tech Jam International, Sep. 26, 2002, specifies a monolithic thermopile sensor array produced using bulk Si micromachining technology. The 64 elements are situated on a chip having a size of 8×8 mm, each element being thermally separated by silicon walls using wet etching technology. The technologically dictated size of the chip leads to relatively high production costs and is once again an obstacle to cost-effective mass-produced applications.
In both the aforementioned solutions, the filling factor is particularly poor.
Besides these thermopile solutions there are further solutions in relation to low-cost infrared arrays.
“A surface micromachined thermopile detector array with an interference-based absorber”, J. Micromech. Microeng. 21 (2011) 8 pp describes a thermopile detector array using surface silicon micromachining. Besides the production of a thermopile, said publication is primarily concerned with a CMOS-compatible interference-based absorber consisting of four layers lying one above another (SiC/Ti/SiC/Al). This layer stack ostensibly absorbs wavelengths in the range of 1-5 μm. However, these wavelengths are not very useful for applications appertaining to person or object detection. Furthermore, problems in the production process are described. Inter alia, residues occur during the removal of the silicon nitride layer and can form a non-transparent film and, in the worst case, can lead to the destruction of the structure.
Skidmore et al.: “Pixel Structure having an umbrella type absorber with one or more recesses or channels sized to increase radiation absorption” US 2009/014017 A1 describe a pixel structure having a so-called umbrella-type absorber. A pixel structure consisting of a bolometer and a substrate is described. The bolometer comprises a transducer having a plurality of holes or channels which are intended to increase the resistance and the absorption. The recesses or channels furthermore have the effect that those parts of the infrared radiation which would otherwise be reflected are directed into the absorber. Said recess also reduces the thermal mass of the bolometer. As a result, however, more process steps are required and the production process becomes more expensive.
The umbrella-type absorber is situated above a bolometer. On account of the construction of said bolometer, however, a vacuum packaging is necessary; in addition, bolometers generally require a temperature stabilization or a shutter or other complicated correction methods in order to compensate for the great drift of the sensitive material.
The vacuum packaging is complicated and expensive above all for reliable applications. The transducer can consist of vanadium oxide (VOx), titanium oxide (TiOx), amorphous silicon or other materials having a good temperature-resistance behavior.
None of these structures described allows the construction of cost-effective thermopile arrays with a high filling level which manage without vacuum.
In all the thermopile infrared array sensor cells described, the absorber area is small in comparison with the pixel area. That limits the maximum achievable signal portion per pixel and increases the risk of incorrect measurements.
The signal voltage of a thermopile pixel can be increased, inter alia, by a plurality of series-connected thermopairs being structured on the pixel. In order to make use of low manufacturing costs, a standard CMOS process in which the thermoelements lie alongside one another has to be used. If the number of thermoelements on the beam is increased, then inevitably the beam is widened—and at the same time the central region with the absorber area becomes even smaller, however, which in turn reduces the quantity of absorbed infrared radiation energy per pixel and thus impairs the filling level even further.